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Sushi knives are sharpened only on one side for similar utility.

Western chefs do a lot of chopping strokes. The 40 degree angle holds up better under those conditions; however, this is also a hold over from when the blades were thicker and the steel not so hard...

It is the total included angle of both bevels that is stated.

Place your razor on a flat surface. Place a ruler on top from blade edge to spine. Check the angle with a high school geometry...